The conjunctiva serves to secrete mucous to aid in lubrication and cleaning of the corneal surface.

PRINCESS NOOR said:

2. does the vitreous humour and the sclera have the same function?

(because i read in one book that the function of the vitreous humour is to maintain the shape of the eye,and i know the function of the sclera is to maintain the shape of the eye too).

You're right that they both take part in the shaping of the eye, but for two different reasons. The sclera in the outer membrane of the eye thus serving to hold everything in. Much of that "everything" is vitreous humor, a viscous fluid that also supports the eye by being bounded by the sclera, if the vitreous humor was not there the sclera would be an empty bag, if the sclera was not there all you would have is a puddle. The vitreous humor may also take place in lens maintenance since a considerable amount of the antioxidant, vitamin C, is found in this tissue, which brings us to....

PRINCESS NOOR said:

3. does the lens consider as a living or non-living?why?

The lens forms from the inside out and is made up primarily of cells containing crystallin proteins. These proteins, while in solution, allow the lens to function as an optically-clear, cell-based structure used to accommodate distance vision. These cells are no longer in contact with the circulatory system and thus have reduced metabolism to little or none, effectively making then non-living. The fascinating thing about these cells and their associated crystallin proteins is their extremely long lifespan. Most proteins live for at most hours or days, but these lens proteins must maintain solubility and optical clarity for the lifetime of the animal. This doesn't always happen and cataracts result which cloud the lens as these proteins breakdown and aggregate. That may be part of the function of the antioxidants found in the vitreous humor, to cut down on oxidative damage in the lens.

Check these websites for more eye/crystallin related materialhttp://webvision.med.utah.edu/
http://www.chemsoc.org/chembytes/ezine/2002/gross_jan02.htm [Broken]